Secondary, lithium-ion cells and batteries are well known in the art. One such lithium-ion cell comprises essentially a carbonaceous anode, a lithium-retentive cathode, and a non-aqueous, lithium-ion-conducting electrolyte therebetween. The carbon anode comprises any of the various forms of carbon (e.g., coke, graphite etc.) which are capable of reversibly retaining lithium species. Graphite is favored by many for its high lithium-retention capacity. Carbon fibers are particularly advantageous because they have excellent mechanical properties which permit the making of rigid electrodes which withstand degradation during repeated charge-discharge cycling. Moreover, their high surface area allows rapid charge/discharge rates. The carbon may be pressed into a porous conductor or, more commonly, bonded to an electrically conductive carrier (e.g. copper foil) by means of a suitable organic binder. The cathode comprises such materials as electronically conductive polymers (e.g., polyaniline, polythiophene and their derivatives) or transition metal chalcogenides which are bonded to an electrically conductive carrier (e.g., aluminum foil) by a suitable organic binder.
Carbon anodes and transition metal chalcogenide cathodes reversibly retain lithium by an intercalation mechanism wherein lithium species become lodged within the lattices of the carbon and chalcogenide materials. In the carbon anode, there is a partial charge transfer between the lithium species and the ".pi." bonds of the carbon, whereas in the metal chalcogenide cathode there is nearly a total charge transfer between the lithium species and the transition metal component of the metal chalcogenide. Chalcogenides known to be useful in lithium-ion cells include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt and manganese with nickel and cobalt oxides being among the more popular cathode materials used commercially.
Manganese oxide has been proposed as a low cost alternative to the nickel and cobalt oxides.
Lithium-ion cell electrolytes comprise a lithium salt dissolved in a vehicle which may be (1) completely liquid, or (2) an immobilized liquid, (e.g., gelled, or entrapped in a polymer matrix), or (3) a pure polymer. Known polymer matrices for entrapping the electrolyte include polyacrylates, polyurethanes, polydialkylsiloxanes, polymethacrylates, polyphosphazenes, polyethers, and polycarbonates, and may be polymerized in situ in the presence of the electrolyte to trap the electrolyte therein as the polymerization occurs. Known polymers for pure polymer electrolyte systems include polyethylene oxide (PEO), polymethylene-polyethylene oxide (MPEO) or polyphosphazenes (PPE). Known lithium salts for this purpose include, for example, LiPF.sub.6, LiClO.sub.4, LiSCN, LiAlCl.sub.4, LiBF.sub.4, LiN(CF.sub.3 SO.sub.2).sub.2, LiCF.sub.3 SO.sub.3, LiC(SO.sub.2 CF.sub.3).sub.3, LiO.sub.3 SCF.sub.2 CF.sub.3, LiC.sub.6 F.sub.5 SO.sub.3, and LiO.sub.2 CF.sub.3, LiAsF.sub.6, and LiSbF.sub.6. Known organic solvents (i.e., vehicles) for the lithium salts include, for example, propylene carbonate, ethylene carbonate, dialkyl carbonates, cyclic ethers, cyclic esters, glymes, lactones, formates, esters, sulfones, nitriles, and oxazolidinones.
Lithium cells made from pure polymer electrolytes, or liquid electrolytes entrapped in a polymer matrix, are known in the art as "lithium-polymer" cells, and the electrolytes therefor are known as polymeric electrolytes. Lithium-polymer cells are often made by laminating thin films of the anode, cathode and electrolyte together wherein the electrolyte layer is sandwiched between the anode and cathode layers to form an individual cell, and a plurality of such cells are bundled together to form a higher energy/voltage battery.
It is known to electrolytically preload carbon anodes with lithium prior to assembling the cells. Such cells are referred to herein as "anode-loaded" cells. Lithium preloading is accomplished by making the carbon the cathode in an electrolysis cell having a nonaqueous electrolyte (akin to the electrolyte used in the battery) and electrolytically loading the lithium therein. Thereafter, the battery is assembled in the fully charged state with an essentially lithium-free cathode material. Anode-loaded cells frequently have some of their lithium content reacted with, and chemically bound or entrapped by, the carbon so as not to be available for reversible intercalation with the cathode material.
Another known approach to manufacturing lithium-ion cells having carbon anodes is to preload the lithium-retentive cathode material with all of the lithium the cell requires and then to assemble the cell with a carbon anode having little or no lithium in it. Such carbon-anode, lithium-ion cells assembled from lithium-retentive cathodes which have been preloaded with lithium are hereinafter referred to as "cathode-loaded" cells. In the case of transition metal chalcogenides, preloading of the cathode is preferably accomplished during the manufacture of the metal chalcogenide material itself, since it has been found that lithium-metal chalcogenides (e.g., LiMn.sub.2 O.sub.4) can be manufactured having a crystal structure more desirable for intercalating larger quantities of lithium species than chalcogenides first loaded with lithium, in situ, during charging of the cell against an anode preloaded with lithium.
Unfortunately, "cathode-loaded" cells have some problems. In the first place, cathode-loaded cells are inefficient because some of the initial capacity of the cell (i.e., as measured by the amount of lithium preloaded into the cathode) is lost during the first charge-discharge cycle of the cell because it is not thereafter available for subsequent reversible interaction with the electrodes. As a result, it has become common practice to provide cathode-loaded cells with excess preloaded cathode material when the cell is first assembled in order to compensate for the amount of lithium expected to be lost (i.e., rendered irreversible) in the first cycle. This, of course, results in a cell having electrodes which are stoichiometrically unbalanced, as far as their relative reversible lithium retention capability is concerned, since after the first cycle more lithium-retention capacity (i.e., more cathode material) resides in the cathode than is needed to accommodate the reversible lithium species intercalated in the carbon anode. Such excess cathode material adds to the size, weight and cost of the cell. In the second place, cathode-loaded/carbon cells tend to evolve gas during the first cycle of the battery incident to decomposition of the electrolyte's solvent. Such gassing not only produces a combustible gas, but can cause delamination of laminated electrodes in full assembled cells, swelling of sealed cells, separation of the active material from its metal substrate current collector and depletion and contamination (i.e., by reaction byproducts) of the electrolyte all of which contribute to increase the cells internal resistance. The aforesaid problems are particularly troublesome in larger batteries such as might be used to propel an electric or hybrid-electric vehicle.
The problem of excess cathode material in cathode-loaded cells may in part be due to certain inefficiency inherent in the cathode material itself. Hence, for example, some chalogenides (e.g., vanadium oxide) have a very high (nearly 100%) first cycle efficiency (i.e., against deactivated carbon). On the other hand, Li-manganese oxide cathodes have only about 90%-95% first cycle efficiency (i.e., against deactivated carbon) owing to an inherent inability of the manganese oxide cathode to give up all of its lithium to the anode.